bug#65009: 29.1; should emacsclient check BROADWAY_DISPLAY as well as WAYLAND_DISPLAY and DISPLAY?

["Trent W. Buck" <trentbuck@gmail.com>]

I'm writing this from Emacs 28.2 still, but this is about pgtk 29.1.
I haven't actually tested pgtk myself yet (sorry!)

The good folks of #debian-emacs noticed that while emacsclient doesn't
link to any GUI libraries, it does check GUI environment variables.

    https://sources.debian.org/src/emacs/1:29.1+1-2/lib-src/emacsclient.c/?hl=1703#L631-L636

    631  #ifdef HAVE_PGTK
    632        display = egetenv ("WAYLAND_DISPLAY");
    633        alt_display = egetenv ("DISPLAY");
    634  #else
    635        display = egetenv ("DISPLAY");
    636  #endif

This (maybe) causes weirdness when Debian builds several versions of
/bin/emacs, but a single shared version of /bin/emacsclient.

But THIS bug is about what happens if you're running emacs inside the
browser, using GTK's built-in HTML5 backend:

    # Start the display (a.k.a. web server)
    broadwayd :0 &

    # Connect from a client (could be a different container)
    firefox http://127.0.0.1:8080

    # start some GUI apps
    GDK_BACKEND=broadway BROADWAY_DISPLAY=:0 gtk3-demo &
    GDK_BACKEND=broadway BROADWAY_DISPLAY=:0 gnome-terminal -- emacs -nw &
    GDK_BACKEND=broadway BROADWAY_DISPLAY=:0 virt-manager &
    GDK_BACKEND=broadway BROADWAY_DISPLAY=:0 emacs &

AIUI pGTK makes Emacs a fully native GTK app and this Just Works.
But "emacsclient --create-frame" won't work until/unless it checks $BROADWAY_DISPLAY, right?

And while emacs daemon can tell from just
"-display ${WAYLAND_DISPLAY:-$DISPLAY}"
when wayland is wanted (because WAYLAND_DISPLAY starts with "wayland-"),
that's *not* possible with broadway.
So some kind of additional logic is needed on the server side, too?
Or maybe -display :0 will Just Work as long as emacs --daemon also has GDK_BACKEND=broadway?
I don't fully understand what happens inside the GTK layer.

My main use case for this is to have a GUI when running Emacs in Debian GNU/ntoskrnl (a.k.a. WSL1).
That way I have full, completely normal GNU userland (unlike NTEmacs), but
Indic scripts render properly (unlike Debian GNU emacs -nw inside Microsoft Terminal).
(The normal ways to solve this are WSL2 or MSYS2, but I like being weird.)

If the general consensus is "too hard; WONTFIX", I am OK with that.
This is something I want a couple of times a year, not every single day.